August 1, 2006

I do enough public speaking and work with enough clients that I often hear the same questions repeated. Sometimes, as soon as someone starts asking a question, I already know what they are going to ask about. One particular issue that seems to always come up is around the subject of transparency. And the thing is, the people who ask these questions really do understand what transparency is, how it works, and what it's all about. But it's one of those things that occur only when certain planets are aligned, and so it seems as if it's inconsistent behavior.

My goal here today is to make it all crystal clear. Before I even get started by the way, I think it's important to state that today's lesson covers not just Illustrator, but InDesign and Acrobat as well. All of the concepts covered in this post apply to all three apps (hey, we may even squeeze Quark in at the end -- that's me -- always doing the unexpected).

First, let's lay out the facts:

You create a logo that contains two spot colors (in this case, I've used PMS Blue 072 and PMS Red 032). The logo has a drop shadow behind it, and you've correctly set Illustrator's Drop Shadow feature to use the Blue 072 spot color, not black. On Illustrator's artboard, the logo appears correct (see below).

Then you save the art as a PDF/X-1a file because it will be used in an ad and you want to make sure that it will print correctly. Or you save your document using Acrobat 4 (PDF 1.3) compatibility. Alternatively, you save your file as an EPS because maybe you're required to place this logo into a QuarkXPress document. (By the way, if you're saving your file as EPS and you're placing it into InDesign, shame on you, and read this.)

The point here to focus on is that you're saving your file to a flattened format. (If you're unfamiliar with transparency flattening, you can get some information here and here.)

The "problem" that arises is that when you open the PDF in Adobe Acrobat, or Adobe Reader, or when you place the file into QuarkXPress or InDesign and print the file to your laser or inkjet printer, it comes out looking incorrect -- with a white box where the transparent effect should be (see below).

But that white box isn't what causes the head scratching and the frustration. Because when you choose to print as separations (many designers are trained to do this, to check that the plates appear correct before sending out to the printer), the color-separated plates -- in this case, two of them -- appear correct. And the totally wacky thing is that sometimes, when you fiddle with enough settings, you find that the file DOES print correctly, even when you print a composite proof. So what gives? Why does it appear to be so inconsistent?

So this is what I refer to when I say that the "planets need to be aligned" in order for this phenomenon to occur. The key items to focus on here are that you have used a transparent effect, and you've used a spot color. Let's discuss:

When you have a transparent effect, the result is a mixture of the inks. In this case, the shadow, which is PMS Blue 072, blends right into the PMS Red 032 background. By default, when one color sits on top of another color, a knockout occurs -- meaning the area beneath the top shape is removed from the lower object. Otherwise, the top color will print on top of the bottom color when the paper is run through the printing press, causing the two inks to mix. In our case of the red and blue colors, the result would be purple in appearance.

However, in this case, where you WANT the drop shadow to blend into the background on press, you have to override that knockout by specifying something called an overprint. An overprint is a command that tells the printer not to remove the background colors that appear beneath the object that appears on top. If you're familiar with overprints and knockouts (or are a trapping freak), this is a simple concept. For those who need more information, Chapter 11 in the book covers it in detail).

The thing is, Illustrator already knows this, and so no action is required on your part. When you print your file from Illustrator, all of these settings are done automatically, so your file looks great when you print it -- either as a composite, or as separations. The same applies when you save your file from Illustrator as a native Illustrator file and place it into InDesign, or if you create a PDF with Acrobat 5 compatibility (PDF 1.4) or higher.

But when you save your file to a format that doesn't support transparency, Illustrator has to flatten the transparency. And in that process, Illustrator realizes that in order to preserve the spot colors so that they print in separations correctly, the drop shadow must be set to overprint the background color.

The problem is that overprint commands are only honored when you print your file as separations. When you are previewing your document on screen, or when you are printing a composite proof of your file, the overprint commands aren't used and the result will be white where overprinting should occur. The file will print correctly when you print as separations, because at that time, the overprints are honored (as they should be).

So now we know WHY we're seeing the white boxes when we view a PDF/X-1a in Adobe Reader, or why we see them when printing a composite proof from QuarkXPress or InDesign (when EPS is used). And here's how to solve the issue:

If you're opening a PDF 1.3 file in Adobe Reader, and you want to see the file correctly on your screen, you need to turn on a preview feature called Overprint Preview. Both Illustrator and InDesign have this feature (found in the View menu), but they've managed to do a good job hiding this feature in Acrobat: open Preferences and navigate to the Page Display panel. Then check the button marked Overprint Preview. The file will now appear correctly on screen.

The final issue here is to find a way to have the file appear correct when you print a composite proof. To do that, you can call on a feature that Adobe applications have in their Print dialog boxes that allow the apps to simulate the appearance of the overprint in a composite print.

If you're in Acrobat, choose Print and then click on the Advanced button. Then check the box marked Simulate Overprinting in the Output section of the dialog (see below).

If you're printing the file from InDesign (and you've used EPS for the Illustrator file), choose File > Print, navigate to the Output panel of the Print dialog box and check the box marked Simulate Overprint (see below). This option will only be available when printing a composite of course. If you placed the Illustrator file as a native .ai file, you don't have to worry about anything as InDesign will automatically print the file so that it appears correctly in both composite and separations form.

If you're working on a file with overprints in Illustrator and need to simulate those overprints when printing from Illustrator, you can do so by choosing File > Print, navigating to the Advanced panel of the Print dialog, and choosing the Simulate option in the Overprints popup menu (see below).

If you're ever playing Trivial Pursuit: Adobe Edition (now there's an idea), the Preserve option keeps any overprints in the file intact (the default setting), and the Discard option ignores all overprint settings in the file (useful for when you want a 3rd party trapping utility or a RIP's internal trapping software to decide what overprints and what doesn't).

To close, if you're using QuarkXPress, you really don't have an option, as that program doesn't allow you to simulate overprint commands when printing composite proofs. One workaround is to create two versions of your file: one that uses spot colors which will separate correctly when you print separations, and one version where you've converted your spot colors to process colors. When you convert to process, the overprints aren't needed and the file will print with the correct appearance on a composite proof. Oddly enough, InDesign has a feature called Ink Manager which contains a checkbox that will convert all spot colors to process at print time -- even when printing a composite. This will also make it possible to preview the file correctly when printed from InDesign. I am not aware of such a feature in Quark (Quark only has that setting available when you print separations, not composite).

"But when you save your file to an unflattened format, Illustrator has to flatten the transparency. And in that process, Illustrator realizes that in order to preserve the spot colors so that they print in separations correctly, the drop shadow must be set to overprint the background color."

Would that not be...

"But when you save your file to a flattened format.

A permanent link to this would be nice in the Illustrator faq forum.

Have you noticed a increase, decrease or is the amount still the same for the number of times this question is asked.

I could not resist and posted a link in the Illustrator Mac forum and will do shortly in the Win side. I hope you do not mind. So, how about deleting my earlier post, except for "The Book" link? Since it is defunct info and it would hide my embarrassing a / an rule mistake. : )

I work in a digital printshop printing on a Xerox IGen, so I always print composite output. We run into this problem frequently, and our usual fix is to convert the spot colors to CMYK. Do you have any other recommendations?

hello,i have some trouble with illustrator file from my client,in the file there is a block color on design in preview. but when i change to "overprint preview" the block color is became transparent, blend with below layer. and the layer block is lost when i export to photoshop.

the trouble is i want the block not to blend when overprint preview, just simply block color.

An Illustrator file: the front of pack - on 1 layer it has a radial gradient of a spot colour yellow (gradient to white)with a transparency setting of 'multiply' over another layer of a radial gradient of varying shades of a CMYK yellow (different %). This is so it looks like a sunburst. The sides and back of pack are pure spot colour yellow. Printer tells me it won't work - that front and back of pack will be two different shades of yellow!

This is crazy, I have always used Illustrator 10, with my old mac, I am a screenprinter so want black opaque on the film, when I print on my epson 1290 inkjet, it works perfect, so on my newer mac I have illustrator CS2, I have converted, the black to a spot colour, (which you do not have to do on illustrator 10) it works okay on paper, prints it black as its on the screen, but when I switch to putting on film, I use the pull down, on the output to say I am using film, guess what the black comes out brown, anybody please help me!